Farewell to a native species dying in the lake?

Older Kakahi/freshwater mussels show only sluggish growth – with almost no juveniles to replace them.

By Martin Freeth

Freshwater kākahi – native mussels once a key food source in Lake Wairarapa – face potential extinction as they pay the cost of urban discharges and farm run-off. 

Lake Wairarapa’s supertrophic water quality is pushing kākahi towards extinction and the ecologic future of the lake and its precious wetlands is highly uncertain despite the efforts of various stakeholders.

(Supertrophic: a trophic level greater than 5 means water quality is rated “very poor.” Lake Wairarapa is saturated in phosphorus and nitrogen and has poor water clarity. It has been officially classified as supertrophic since 2012).

Kahungunu Iwi, the lake’s kaitiaki/guardians, told The Star earlier this year that “the lake is dying.”

Urban discharges and farm run-off have long been recognised as causing serious decline in the water quality of Wairarapa Moana – the North Island’s third-largest freshwater body and part of the globally-recognised Wairarapa Moana lakes, wetland and river environment.

The science-based, official LAWA (Land, Air, Water Aotearoa) website records Lake Wairarapa as supertrophic because of its persistent high nitrate and phosphate readings, and levels of algae and sediment. 

There’s been little change since water quality monitoring started in 1994 – despite a Water Conservation Order placed on the lake for its ecological value and despite a 2020 declaration of

Wairarapa Moana’s international importance under the Ramsar (UN) Convention on wetlands.

Kākahi are a key bioindicator of ecological health and the plight of this species is highlighted in a new research report from NIWA marine and freshwater ecologist Mark Fenwick after 10 years’ systematic monitoring at two Lake Wairarapa sites.

The research has shown relative abundance of older kākahi but their growth is slow and there’s an almost complete lack of juveniles in the monitored populations. 

“It is possible that the effort required by adults to extract food from the sediment rich water leaves them without the necessary strength for growth and reproduction,” Fenwick said in a report.

Fenwick’s finding that the shellfish are headed into “functional extinction” – published during Conservation Week (2-8 September) – comes as South Wairarapa District Council (SWDC) continues struggling to attain Greater Wellington Regional Council (GWRC) consent for upgrading work on its Featherston wastewater treatment plant. 

Since 1975, the plant has been discharging all its treated human effluent into Donald’s Creek, thence into the north-eastern end of Lake Wairarapa. (Note that it’s “treated” effluent – not raw).

The Featherston plant has been operated under successive extensions to a resource consent that expired in 2012. SWDC has so far invested more than $9.5 million in land purchases and investigations work towards an eventual upgrade that will see much of the treated effluent disposed to land, and perhaps also into a man-made wetland.

SWDC‘s new consent application includes plans for a “clarification system” to reduce sediment and contaminants from wastewater coming from its oxidation pond and “a constructed wetland and revegetated area” surrounded by concrete walls for filtering the water prior to it discharging into the creek. It would also trial the final wastewater disposal through trickle irrigation onto SWDC land.

The Wellington Water-prepared application makes modest claims about 10-year environmental benefits, though “the proposed clarification process … is expected to significantly improve wastewater quality through removal of sediment and nutrients including nitrogen and phosphorus.”

Wellington Water acknowledges risks to human health from microbial contamination of water supply wells when treated wastewater is irrigated to land.

There are also huge environmental and technical challenges around the Featherston plant because of the area’s high natural water table and substantial ground water infiltration into the town’s sewer pipes.

The company has put the upgrade costs for Featherston at an estimated $21 million by December 2027. Further cost is expected for not-yet planned and budgeted work on the Featherston system.

Meantime, SWDC’s wastewater plants for Martinborough and Greytown, both discharging some of their treated effluent into the Ruamahanga River, also await major upgrading works with complex environmental considerations and high cost to ratepayers.

Farm run-off another focus

Farm run-off issues across Wairarapa are starting to be addressed, largely by Greater Wellington Regional Council under its regulatory framework in high priority catchment areas and through grants for riparian planting, de-stocking sensitive land and wetland restoration.

Otukura is the priority catchment closest to Wairarapa Moana where 123 farms or horticulture blocks were facing a GWRC deadline to have certified Farm Environment Plans (cFEPs) in place by 30 September.

GWRC has now told The Star only six met this regulatory obligation though others have already been taking “serious” on-farm actions to reduce their freshwater and environmental impacts.

GWRC Ecosystems and Community manager David Boone said council is helping Otukura properties to complete and implement cFEPs _ with enforcement an option “if really needed.”

GWRC is one of several regional councils which now require farm plans in specific catchments – despite the coalition government in October pausing a separate nationwide roll-out of of freshwater farm planning regulations because of cost and complexity for farmers.

Boone says whatever the pause outcome, on-farm requirements to identify, manage and reduce farming impacts on natural freshwater will definitely become an integral part of Wairarapa farming and growing.

Otukura catchment spans a large area between the Ruamahanga and Tauherenikau Rivers _ extending both ways across State Highways 2 and 53 (towards Kahutara), and including one large dairy property seen as a national exemplar for sound environmental practice in farming.

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Restoration efforts increasing?

Meantime, GWRC continues to lead the Wairarapa Moana Wetlands Project begun in 2008 with the aim of restoring the ecology through mass planting of native, endemic species back into the wetlands and lake shorelines. This year’s planting plan has included 28,400 more plants in six locations.

The project received a major boost in 2020 with $3.5 million cash funding under the previous government’s Jobs for Nature scheme. But according to the Ministry for Primary Industries, this scheme and its funding flow will end next 30 June. That obviously creates uncertainty over the scale of restoration work from 2025 onwards.

Over-arching all other concerns with the lake and wetlands will be work now started by the Wairarapa Moana Statutory Board – the co-governance body created by the Te Rohe o Rongokako Joint Redress Act 2022. The Act has returned the lake bed and certain reserve lands to the ownership of Rangitāne o Wairarapa and Rangitāne o Tamaki nui-ā-Rua and Ngāti Kahungunu ki Wairarapa Tāmaki nui-a-Rua under their respective Treaty settlements for cultural redress.

The Act also establishes the board – with membership including GWRC and SWDC representatives – “to act as a guardian of Wairarapa Moana and the Ruamahanga River catchment, for the benefit of present and future generations.”

The board’s first priority is to draw up a new Wairarapa Moana Document including plans for natural resource and reserves management. Minutes of the inaugural board meeting (March 2024) show the first step will be creation of “an overarching vision that recognises and provides for the cultural, spiritual and ecological values” of the Moana. This could take 1-2 years to finalise, including processes of public consultation. Uncertainty over the lake and its well-being is set to continue.